1. Field of the Invention
The present invention relates to a manufacturing method of a liquid crystal display unit using a polymeric substrate, and more particularly relates to a method for simply manufacturing a liquid crystal display unit excellent in mass production by using the polymeric substrate.
2. Background Information
FIG. 7 shows the construction of a conventional STN (Super Twisted Nematic) liquid crystal display unit using a polymeric substrate. One example of a manufacturing method of the STN liquid crystal display unit using the polymeric substrate will be explained with reference to FIG. 7. A transparent substrate 22 constructed by ITO is formed on a polymeric substrate 21 constructed by polycarbonate, etc. Next, polyamic acid or polyimide solution is printed and hardened on the polymeric substrate 21 so that an orientation film 23 constructed by polyimide is formed. Next, this substrate 21 is orientated by rubbing this substrate 21 with a buff cloth constructed by cotton and rayon fibers. A transparent electrode 25 constructed by the ITO and an orientation film 26 are similarly formed on another opposite polymeric substrate 24. This substrate 24 is orientated by rubbing such that a rubbing direction of this substrate 24 forms an angle of about 200 to 260° with respect to the rubbing direction of the previous substrate 21. These two substrates are opposed to each other and are integrated with each other by a sealant 27. A liquid crystal 28 is sealed into a clearance of these substrates so that an STN liquid crystal display unit is formed. In the STN liquid crystal display unit using such a polymeric substrate, the substrate is formed by a high polymer instead of conventional glass so that the display unit is not easily cracked and can be made light in weight. Further, since an STN display mode is used, the deterioration of display quality is small even when the number of electrodes operated in time division is increased. A large capacity display can be also realized.
Another advantage of the adoption of the polymeric substrate is that a continuous processing treatment can be taken by so-called Roll to Roll in addition to sheet-fed processing as in a working process treatment of the glass substrate so far since the polymeric substrate is flexible. In the continuous processing using Roll to Roll, an orientation film printing process, an orientation film solidifying process and an orientation process can be continuously performed by using the polymeric substrate 31 wound in a roll shape so that the manufacturing method is very simple and excellent in mass production.
However, the orientation of the orientation film in the STN display mode is a homogeneous (parallel) orientation, and the orientation angles of both the upper and lower substrates range from 200 to 260° and therefore are not constant in accordance with the specification of a product.
Since no rubbing direction for processing is constant in accordance with the product, the rubbing direction of the rubbing buff cloth must be changed to a predetermined angle with respect to a flowing direction of the substrate at each tooling changing time so that it takes much time and labor. A continuous substrate extending in a longitudinal direction is used in the continuous processing using Roll to Roll. Therefore, after all one roll is processed, the operation of a line is stopped and tooling is changed. In the case of the STN liquid crystal display unit, important characteristics such as electro-optic characteristics, an angle of visibility, etc. are determined by the orientation direction. Therefore, it is not too much to say that the orientation direction is changed every product. When the operation on the continuous line is stopped every product, productivity is reduced in comparison with the working process of the glass substrate of sheet-fed processing.
In the rubbing process for obtaining the homogeneous orientation, rubbing force in the rubbing is relatively strong so that the polymeric substrate much softer than glass is damaged. Therefore, there is a case in which a serious defect in display quality is caused.
Further, high optical isotropy is required in the polymeric substrate because of the homogeneous orientation having such an unfixed angle. The reasons for this are as follows. The STN liquid crystal display unit is optically designed such that a polarizing element is arranged outside a cell and linearly polarized light is incident to a liquid crystal molecule. The glass substrate generally used in the liquid crystal display unit is conventionally optically isotropic, and the linearly polarized light incident in any direction is emitted as linearly polarized light in a direction as it is. Therefore, the STN optical design could be made by neglecting the substrate.
However, in the case of the polymeric substrate, refractive indexes in the x and y directions are almost anisotropic, and are optically not isotropic. Namely, the linearly polarized light incident from a direction except for these x and y directions becomes elliptically polarized light at an emitting time from the polymeric substrate so that the optical design of the STN liquid crystal display unit greatly gets out of order. Two solving methods of this problem are considered. One solving method is to remove the optical anisotropy of the used polymeric substrate. However, to remove the optical anisotropy of the polymeric substrate, it is necessary to reduce the optical anisotropy of a material molecule itself, and consider that no material molecule is arranged in one direction at a manufacturing time of the substrate. Therefore, the substrate manufacture becomes very complicated, and a selecting range of the material itself is very limited. Another solving method is to align the orientation direction of a liquid crystal molecule on the substrate, and a phase advancing axis direction or a phase delaying axis direction of the optical anisotropy of the polymeric substrate. If this method is adopted, the linearly polarized light incident to the polymeric substrate does not become elliptically polarized light at the emitting time, and is the linearly polarized light as it is so that no optical design gets out of order. However, in the case of the polymeric substrate having an elongated roll shape, the phase advancing axis direction or the phase delaying axis direction of the optical anisotropy must be set to be parallel or perpendicular to a long side due to a restriction from the manufacturing method of the polymeric substrate. Therefore, no orientation angle can be set to an arbitrary angle from 200 to 260° required in the STN. In contrast to this, when a pattern is arranged in the orientation direction, a pattern utilization rate in the polymeric substrate is greatly reduced and a combination of upper and lower substrates in a later assembly process becomes very complicated.
As mentioned above, the conventional manufacturing method of the display unit using the polymeric substrate has the above problems. Accordingly, it is impossible to simply realize the manufacturing method in Roll to Roll by using flexibility of the polymeric substrate. Therefore, an object of the present invention is to provide a simple manufacturing method having high productivity without damaging the polymeric substrate.